Please follow the referenced bug log for the details, but the problem here is common to all of Debian and wheezy --> jessie or unstable dist-upgrades. It applies to basically every dist-upgrade spanning from around xmas to may, when systemd-sysv effectively became essential without the default being toggled formally; if you ever upgraded between xmas and may, you won't ever notice that problem (but it applies to upgrading fresh installs, of course).

While apt segfaulting is the visible part of it (right now, that will be rectified by the next apt version - however it still won't work seemlessly with a fixed apt, as dpkg will then righfully bail out instead), the actual problem underneath is buggy packaging (dependency crafting) of the sysvinit --> systemd transition. Therefore you first need to bump your installation to a "safe" intermediate version of sysvinit (>= 2.88dsf-44), which makes the "sysvinit" package an empty shell (transitional package) moving its actual contens into a new "sysvinit-core" package, before you can actually continue the dist-upgrade. As this seriously affects Debian's own wheezy --> jessie+ upgrade path, it will have to be fixed (and a fix + formally toggling the default to systemd-sysv is long overdue already).

So all in all, dist-upgrading is working and if you follow the steps depicted above, it's supposed to work out fine (I successfully tested that a few times over the last two weeks), with no harm done if you miss it at first (but you won't be able to escape this trap without explicitly installing the newer, intermediate sysvinit version first). For several reasons, especially with the future upgrade path in mind, afterwards switching to systemd-sysv has become effectively mandatory (at least for any kind of desktop'ish systems) by now. There is no problem with systemd here, which is supposed to be a mostly invisible migration, it's just a broken dependency set preventing a smooth migration for now.